For example, as disclosed in Patent Document 1, a plate-fin heat exchanger is structured such that an independently formed header tank is joined, by welding, to a core formed by stacking a multitude of plates on each other and integrating them together. Specifically, the entire peripheral edge of the opening of the header tank is provided with a bevel inclined from the internal surface of the header tank toward the external surface thereof at a predetermined bevel angle (groove angle), and is welded from the outside of the header tank.